Problem: Solve for $x$ and $y$ using substitution. ${2x+y = -7}$ ${x = -3y-1}$
Answer: Since $x$ has already been solved for, substitute $-3y-1$ for $x$ in the first equation. ${2}{(-3y-1)}{+ y = -7}$ Simplify and solve for $y$ $-6y-2 + y = -7$ $-5y-2 = -7$ $-5y-2{+2} = -7{+2}$ $-5y = -5$ $\dfrac{-5y}{{-5}} = \dfrac{-5}{{-5}}$ ${y = 1}$ Now that you know ${y = 1}$ , plug it back into $\thinspace {x = -3y-1}\thinspace$ to find $x$ ${x = -3}{(1)}{ - 1}$ $x = -3 - 1$ ${x = -4}$ You can also plug ${y = 1}$ into $\thinspace {2x+y = -7}\thinspace$ and get the same answer for $x$ : ${2x + }{(1)}{= -7}$ ${x = -4}$